The hydrotreating process is a dominant process technology in refineries for fuel upgrading and clean-up. The hydrotreating reaction can be classified into four categories: hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodemetallation (HDM), and hydrodeoxygenation (HDO). In many cases, these reactions proceed simultaneously inside the reactor. Among them, HDS is of primary importance. The HDS reaction involves the breakage of C-S bonds by addition of hydrogen molecules so as to release sulfur as H2S gas. C-S bonds often exist inside an aromatic molecular structure, requiring the HDS reaction to be concomitant with aromatic saturation.
Recently enacted U.S. legislation requires a reduction in diesel fuel sulfur level to 15 ppmw by July 2006 for most of the nation""s large refiners. These regulations will force the refining industry to make significant capital investments to increase their HDS capability. Presently available commercial hydrotreating process and catalyst technology may not provide cost-effective solutions to meet this need. The present invention provides monolith-based HDS catalysts and processes that offer significant improvements in conversion efficiency over presently available commercial hydrotreating technology.
Current commercial HDS technology is mature and is based on cobalt/molybdenum impregnated gamma-alumina or on nickel/molybdenum impregnated catalysts. These catalysts are employed in large reactors as random packed beds of spherical, cylindrical, or shaped extrudate beads. HDS reactors typically operate in a trickle-bed mode wherein the raw, high sulfur-containing distillate-range hydrocarbon liquid flows at relatively low velocity downward through the catalyst bed, while a hydrogen-rich treat gas flows co-currently downward through the catalyst at a much higher velocity. Organic sulfur compounds and organic nitrogen compounds in the distillate are converted to hydrogen sulfide and ammonia, which are separated from the treated liquid product downstream in the vapor/liquid separator and in a stripper distillation tower. The sour gas is often treated by amine absorption to remove the H2S and NH3, with the large excess of hydrogen containing gas recycled back to the process. The recycle gas rate is adjusted so as to provide a large excess of hydrogen over the stoichiometric requirements for reaction.
In addition to HDS and HDN reactions, other hydrotreating reactions also can occur in parallel during processing, including saturation of olefin and aromatic compounds. These reactions consume hydrogen and release heat, which tends to increase the capital and operating cost of the process. Much of the research and development activity today for conventional HDS catalysts is aimed at higher volumetric activity for desulfurization and improved selectivity in order to reduce excess hydrogen consumption. However, no major breakthrough HDS technology has been reported that drastically enhances the HDS activity for production of ultra low sulfur (ppm level) diesel fuel production in a cost-effective way.
A review of existing catalyst and process technology options is given by Knudsen, xe2x80x9cCatalytic and Process Technologies for Ultra Low Sulfur Dieselxe2x80x9d, Applied Catalysis A: General, 189 (1999) 205-215. It is generally recognized that a 4 to 5 times enhancement in catalyst activity will be needed for low sulfur diesel production; optimization of conventional catalyst pellets is expected to yield only about a 25 to 100% activity improvement much short of the desired activity. Further activity enhancements may be realized by optimizing process conditions, such as reaction temperature, gas/liquid ratio, reactor pressure, gas flow rate, hydrogen gas purity, etc. All of these improvements are incremental in nature and typically require large capital investment.
References to the concept of utilizing monolithic catalysts for hydrotreating reactions may be found in the literature, although no practical utility has yet been disclosed. For example, CoMo/alumina monolith catalysts have been tested for the HDS and HDN treatment of heavy oil, as reported by D. S. Soni and B. L. Crynes in xe2x80x9cA Comparison of the Hydrodesulfurization and Hydrodenitrogenation Activities of Monolith Alumina Impregnated with Cobalt and Molybdenum and a Commercial Catalystxe2x80x9d, ACS Symp. Ser., 156, 156-207 (1981). The rates of fluid flow through the catalyst in this study ( less than 0.02 cm/sec) are very low and suggest no practical advantage over conventional pelletized catalysts. A further investigation of the HDS reaction over a CoMo/alumina monolith catalyst in a model reactor is reported by S. Irandoust and O. Gahne in xe2x80x9cCompetitive Hydrodesulfurization and Hydrogenation in a Monolithic Reactorxe2x80x9d, AIChE Journal, 36 (5), pp 746-752 (1990). However, this kinetic study involved only the conversion of a simple hydrocarbon feed in a laboratory bench reactor at high recycling rates and low conversions per pass. Again, these are conditions that are of no practical interest for the commercial hydrotreating of complex refinery distillates, gas oils, or the like.
The present invention involves the use of a monolithic catalyst reactor to achieve high one-pass conversion rates in the hydrotreating of complex oil streams under reaction conditions of practical utility for commercial HDS processing. We have found that catalyst and reactor efficiency in the HDS process can be substantially improved by taking advantage of the unique parallel channel geometry of monolithic catalysts of honeycomb configuration. Monolithic honeycomb catalyst and reactors employing them operate in a fundamentally different way from conventional pellet catalysts and trickle bed reactors incorporating randomly packed catalyst beds. By uniformly distributing gas and liquid to each channel, the issues of partial wetting and stagnant fluid areas are significantly reduced, and overall catalyst volumetric efficiency is increased.